Trucks of this type have been known. A typical example of such truck is an automatic vertically movable working truck disclosed in Japanese Utility Model Publication No. 54-16427. According to this prior art arrangement, the truck comprises a rectangular frame-shaped body, wheels carried by the body at four corners thereof, and a support structure extending upward from the rear end of the body. There is provided a bed for placement of goods which is vertically movable by being guided along the supporting structure, with a pair of chains arranged between the bed and the upper end of the supporting structure via stationary sprockets. Each chain is trained around a moving sprocket mounted on a hanger plate, and between the hanger plate and the body there are arranged a plurality of springs and a locking hydraulic cylinder.
According to this prior art arrangement, the bed is suspended through the intermediary of the springs and, therefore, when no goods are placed on the bed, the bed is lifted to a level adjacent the upper end of the supporting structure by the restoring force of the springs. When goods are placed on the bed, the hanger plate is moved upward against the contracting force of the springs under the tension applied to the chain, so that the bed is lowered in response to the load. Such movement is effected while the hydraulic cylinder is opened, and when the hydralulic cylinder is closed, the bed is caused to stop at its present position irrespective of the load.
According to the above described conventional arrangement, the upper ends and lower ends of the springs are fixed respectively to the hanger plate and the body. Therefore, the arrangement cannot make effective adaptations to changes in load. Where goods to be handled are always same in weight, there is no problem; but if lightweight goods are placed on a bed adapted to heavier goods by a larger spring constant of springs, the bed will not be sufficiently lowered by the lightweight goods. On the other hand, if heavier goods are placed on a bed adapted to lightweight goods by a small spring constant of springs, the bed will reach the lowermost position at a stroke and will remain in that position until the bed is relieved of a fair amount of the load. In either case, workers find inconvenience in the handling of goods. This inconvenience may be removed by changing the number of springs to be put in the operative condition, but this involves removal and remounting of the cover for the supporting structure, which is very troublesome. Another problem is that one will be at a loss how to deal with the springs put in the inoperative condition.
When such conventional type of truck is in operation, the number of springs in operative condition cannot be visually grasped, and the operator cannot easily know the suitable number of springs to be put in the operative condition for handling a given amount of the load.
Further, according to the prior art arrangement, the truck is subjected to forward or backward motion by a push or pull given at a handgrip disposed on the back of the supporting structure at an upper portion thereof. In either case, the rectangular body of the truck is an obstacle to the ambulation of an operator who is giving a push or pull at the heavy-laden truck. Another trouble is that when an operator is pulling the truck with his back facing the truck, his heel may become positioned under the rear edge of the body of the truck, with the result of the Achilles' tendon being damaged.
Guide devices for trucks of this type have also been known. A typical example is disclosed in Japanese Patent Publication No. 50-1062. In this publication there is described an arrangement comprising a pair of support structures mounted upright on a body having wheels, a pair of side plates, right and left, disposed in the support structures, and a bed for placement of goods which are guided upward and downward along the side plates through guide rollers. The guide rollers are mounted on guide plates integral with the bed, and include upper guide rollers abutting the rear surfaces of the side plates, lower guide rollers abutting the front surfaces of the side plates, and a pair of side guide rollers, upper and lower, abutting the inner surfaces of the side plates. Between the bed and the support structure there is disposed a lift mechanism comprising springs, rollers, and guide bars for guiding the bed upward and downward in response to the weight of the load.
In such arrangement of the conventional type, vertical movement of the bed is carried out while the bed is guided by the side plates via the guide rollers. Such prior art guide device requires a large number of rollers (eight in the embodiment shown) for vertically guiding the bed, and also guide plates for roller mounting.